/usr/include/libfilezilla/ is in libfilezilla-dev 0.4.0.1-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/libfilezilla/apply.hpp
/usr/include/libfilezilla/event.hpp
/usr/include/libfilezilla/event_handler.hpp
/usr/include/libfilezilla/event_loop.hpp
/usr/include/libfilezilla/file.hpp
/usr/include/libfilezilla/glue/
/usr/include/libfilezilla/glue/wx.hpp
/usr/include/libfilezilla/iputils.hpp
/usr/include/libfilezilla/libfilezilla.hpp
/usr/include/libfilezilla/local_filesys.hpp
/usr/include/libfilezilla/mutex.hpp
/usr/include/libfilezilla/optional.hpp
/usr/include/libfilezilla/private/
/usr/include/libfilezilla/private/defs.hpp
/usr/include/libfilezilla/private/visibility.hpp
/usr/include/libfilezilla/private/windows.hpp
/usr/include/libfilezilla/process.hpp
/usr/include/libfilezilla/recursive_remove.hpp
/usr/include/libfilezilla/shared.hpp
/usr/include/libfilezilla/string.hpp
/usr/include/libfilezilla/thread.hpp
/usr/include/libfilezilla/time.hpp
/usr/include/libfilezilla/util.hpp
/usr/include/libfilezilla/version.hpp